Monoclonal antibodies will be used to analyze certain processes in mammalian development. Migration: Evidence is presented that there are common features in the migration of primordial germ cells and hematopoietic stem cells, and that the specificity of these migrations resides in the interactions of these cells with the surfaces over which they migrate. Monoclonal antibodies against the cell surfaces of wild-type mice made by the fused spleen cells of those mice with defective migration (Steel, Dominant white spotting) may enable us to identify cell surface molecules essential to these pathways. Differentiation: Evidence is presented that the induction of metanephric tubules by the ureteric bud or neural tube is due to cell-cell interactions. Hybridomas would be made from the spleen cells of those mice (injected with syngeneic spinal cord) whose sera inhibited tubulogenesis in vitro. The monoclonal antibodies could likewise be tested after being screened for their ability to bind to dorsal spinal cord and other inducers of tubulogenesis. In this way, one could determine if different organ systems used similar antigenic stimuli to promote differentiation.